An essential characteristic of MC-CDMA type transmission techniques is to compensate for the loss of data rate caused by duplicating symbols with an increase in data rate obtained by stacking a plurality of symbols from different users over the same time interval or over the same carrier frequency. FIG. 1 illustrates this effect on stacking symbols from different users over time and frequency axes, the stacking being obtained by spreading in the time and frequency domains. Furthermore, a characteristic that is common to multi-carrier techniques is a reduction in bit rate for each sub-carrier while providing a higher transmission bit rate by using these sub-carriers simultaneously. The frequency band is subdivided into small ranges, each allocated to a respective different sub-carrier. The sub-carriers are mutually orthogonal. This property is obtained by the sub-carriers being spaced apart relative to one another by a multiple of the reciprocal of the symbol duration. A sub-carrier modulation system can serve to provide immunity against a selective frequency fade occurring during transmission over the channel, because not all of the sub-carriers are subjected to said fading simultaneously. Nevertheless, in order to counter the fading phenomenon, the transmission channel must nevertheless be estimated and corrected for each sub-carrier on reception of the transmitted signal.
An MC-CDMA transmitter performs various processes on incoming high bit rate binary data in order to generate a so-called MC-CDMA signal that is transmitted over the channel. Thus, the high bit rate binary data is coded, e.g. using a convolution code, and it is modulated, e.g. using modulation of the xPSK type (x phase shift keying) or of the xQAM type (x quadrature amplitude modulation), with it being possible for coding and modulation to differ between users. The modulated data is then converted into parallel form as a plurality of modulated data streams at low bit rate. Time spreading is obtained firstly by duplicating each low bit rate modulated data unit, referred to as a “symbol”, so as to obtain SFt symbols from a given low bit rate modulated symbol, and multiplying each symbol by a time spreading code element (referred to as a “chip”), and secondly by paralleling these various weighted symbols in the time domain so that they are typically modulated by sub-carriers in respective different MC-CDMA symbols. FIG. 2 is an illustration thereof in the time-frequency spreading plane. Each initial stream is thus duplicated to give SFt streams of symbols that result from time spreading. In the illustration, the time-spread symbols are distributed respectively over successive different orthogonal symbols. Spreading in the frequency domain is obtained by duplicating several times over each of the symbols that result from time spreading, in order to obtain a total of SFf symbols, by multiplying each of the SFf symbols respectively by a frequency spreading code chip, and by sharing them over SFf sub-carriers of an orthogonal modulator. The modulator generates MC-CDMA orthogonal symbols. After time and frequency modulation, each spread symbol that modulates sub-carrier p, where p=0, 1, . . . , Nc−1, and where Nc is the number of sub-carriers, can be written using the following condensed form:xp,n(k)=d└p/SFƒ┘,n/SFt(k)×c(p mod SFƒ),(n mod SFt)(k)  (1)with:
dmj (m=0, 1, . . . , Nc/SFf−1; j=0, 1, . . . , Nd/SFt−1) the modulated data or symbols;
ciq (i=0, 1, . . . , SFf−1, q=0, 1, . . . , SFt−1) the two-dimensional code sequence;
└z┘ the greatest integer value less than or equal to z, i.e. the integer portion of z;
k the user index; and
n the index of the MC-CDMA symbol, n=0, 1, . . . , Nd−1, where Nd is the number of MC-CDMA symbols in a frame.
The spreading factor SF is expressed in the form SF=SFt×SFf with SFt and SFf being the spreading values respectively in the time domain and in the frequency domain. The number of input data items modulated in a frame is given by the relationship:(Nc×Nd)/(SFt×SFƒ)
The multiplexer multiplexes the spread symbols belonging to different users for a given MC-CDMA symbol n and a given carrier p. This multiplexing function can be expressed by the relationship:
                              x                      p            ,            n                          =                              ∑                          k              =              0                                                      N                u                            -              1                                ⁢                      x                          p              ,              n                                      (              k              )                                                          (        2        )            
The multiplexer positions the symbols xp,n at the input to the inverse discrete Fourier transform modulator (IDFT) in register with the sub-carriers corresponding to their respective indices p.
The symbols xp,n are converted into an MC-CDMA orthogonal symbol by frequency division obtained by the inverse discrete Fourier transform with Nfft points. The duration of an MC-CDMA symbol is thus given by:Td=Nfft×Tfft where Tfft is the inverse of the inter-carrier spacing.
The resulting MC-CDMA orthogonal symbols are converted into analog form by a digital-to-analog converter (DAC) to generate an MC-CDMA signal and they are converted by a frequency up-converter (U/C) to a radio frequency (RF) band for transmission over the transmission channel.
Typically an MC-CDMA system transmits NFFT symbols xp,n during the nth MC-CDMA orthogonal symbol period over Nfft sub-channels determined by sub-carriers at a spacing of ΔF=1/Tfft.
To combat inter-symbol interference (ISI) and inter-carrier interference (ICI), a guard interval (GI) of Ng points such as a cyclic prefix (CP) or zero padding (ZP) is added to each MC-CDMA orthogonal symbol. This guard interval makes it possible to ensure that MC-CDMA orthogonal symbols do not overlap, providing the transmission delay is shorter than the duration of this interval. The total duration Tg of the guard interval is equal to:Tg=Ng×Tfft where Tfft is the duration of a sample of the FFT, given that the duration of an MC-CDMA orthogonal symbol is equal to:Td=Nfft×Tfft The MC-CDMA signal is then expressed in the following form:
                                          s                          (              k              )                                ⁡                      (            t            )                          =                              1                          Nfft                                ×                                    ∑                              n                =                0                                            Nd                -                1                                      ⁢                                          ∑                                  p                  =                  0                                                  Nfft                  -                  1                                            ⁢                                                x                                      p                    ,                    n                                                        (                    k                    )                                                  ×                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                                                                  p                        ⁡                                                  (                                                      t                            -                                                          n                              ⁡                                                              (                                                                  Nfft                                  +                                  Ng                                                                )                                                                                                              )                                                                    /                      Nfft                                                                      ×                                  u                  ⁡                                      (                                          t                      -                                              n                        ⁡                                                  (                                                      Td                            +                            Tg                                                    )                                                                                      )                                                                                                          (        3        )            in which expression the term u(t) is the gate function defined over the interval [0, Td+Tg] and xp,n is the component relating to user k of the transmitted signal that modulates the pth sub-carrier of the nth MC-CDMA symbol.
Assuming that the transmission channel is made up of P discrete paths each characterized by a respective amplitude and delay, the impulse response in baseband of the channel for user k can be expressed in the form:
                                          h                          (              k              )                                ⁡                      (                          t              ,              τ                        )                          =                              ∑                          p              =              0                                      P              -              1                                ⁢                                                    α                p                                  (                  k                  )                                            ⁡                              (                t                )                                      ·                          δ              ⁡                              (                                  τ                  -                                      τ                    p                                                  )                                                                        (        4        )            with αp and τp being respectively the complex channel gain and the delay of the pth path referred to as the spread delay. The total power transmitted by the channel over the various sub-carriers is normalized to 1. It is also assumed that:τmax=max τj<(Td+Tg)equals the duration of an MC-CDMA symbol. This relationship is satisfied by adapting the duration of the guard interval to the characteristics of the transmission channel.
The channel is assumed to be modeled by a finite impulse response (FIR) filter of order L−1, where L is the number of time samples corresponding to the maximum delay τp. The transfer function H(k)(t,f) of the channel for user k can be expressed in the frequency domain in the following form:
                                          H                          (              k              )                                ⁡                      (                          t              ,              f                        )                          -                              ∑                          p              =              0                                      p              -              1                                ⁢                                          ⁢                                                    α                p                                  (                  k                  )                                            ⁡                              (                t                )                                      ·                          exp              ⁡                              (                                                      -                    j2                                    ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  f                  ⁢                                                                          ⁢                                      τ                    p                                                  )                                                                        (        5        )            
The coefficients αp(k) of the impulse response are estimated using known methods that rely on making use of pilot symbols.
On reception, the signal received by a receiver is filtered by a bandpass filter (BPF) and is down-converted (D/C) into baseband. The signal is sampled by an analog-to-digital converter (ADC). After suppressing the guard interval, the sampled data is fed to a discrete Fourier transform (DFT) with Nfft points and is demultiplexed into as many branches as there are sub-carriers.
After demultiplexing, the component modulating the pth sub-carrier of the nth received OFCDM symbol is expressed in the following form:
                              y                      n            ,            p                          =                              1                          Nfft                                ·                                    ∑                              i                =                0                                            Nfft                -                1                                      ⁢                                                  ⁢                                          r                                  n                  ,                  i                                            ·                              ⅇ                                                      -                    j2                                    ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  p                  ⁢                                                                          ⁢                                      ⅈ                    /                    Nfft                                                                                                          (        6        )            
When the maximum of the delays τp (spread delay) is less than the guard interval, the component modulating the pth sub-carrier of the nth received OFCDM symbol can be expressed in the form:yp,n(k)=hp(k)×xp,n(k)+np,n(k)  (7)
To reduce the effects of channel distortion, which can give rise to a loss of orthogonality between different users, it is known to compensate the channel in the frequency domain sub-carrier by sub-carrier, using coefficients Gp(k) that are determined in application in particular of a minimum mean square error (MMSE) criterion, an equal gain combining (EGC) criterion, or a zero forcing (ZF) criterion. Thereafter, for each branch, the data is de-spread, demodulated, and decoded. This compensation amounts to multiplying the received signal by the reciprocal of the channel gain for a given sub-carrier. This compensation can be performed by the user taking account of the coefficients of the channel as given by the following relationship (8):
                              G          p                      (            k            )                          =                  {                                                                                                                                        H                        ^                                            p                                                                        (                          k                          )                                                *                                                                                                                                                                                          H                            ^                                                    p                                                      (                            k                            )                                                                                                                      2                                                        ⁢                                      (                    ZF                    )                                                                                                                                                                                          H                        ^                                            p                                                                        (                          k                          )                                                *                                                                                                                                                            H                          ^                                                p                                                  (                          k                          )                                                                                                                            ⁢                                      (                    EGC                    )                                                                                                                                                                                          H                        ^                                            p                                                                        (                          k                          )                                                *                                                                                                            Nu                        ·                                                                                                                                                                        H                                ^                                                            p                                                              (                                k                                )                                                                                                                                          2                                                                    +                                              σ                        n                        2                                                                              ⁢                                      (                    MMSE                    )                                                                                                          (        8        )            
This method has the drawback of increasing the noise level on reception, in particular when the channel gain value is small, which configuration is typically encountered during fading.